This practical is in two sections – simple distillation and paper chromatography of inks. You can carry out the practical in one session or two, depending on the length of your lesson. Many of your students will have carried out these practicals as part of their Key Stage 3 course so this activity can be used to revise the techniques as well as introduce the additional theory required for chromatography.
FOR THE CHROMATOGRAPHY PART OF THE PRACTICAL:
Questions you could ask to enhance learning and focus your students on important aspects of the practical:
- Why is it important to draw the lines and write labels on the chromatography paper in pencil and not in ink?
- Why should the spots of ink be above the level of the solvent in the beaker?
- What is meant by the term ‘solvent front’?
- What would happen if you used permanent ink instead of water soluble ink? How could you overcome this problem?
- Which is the mobile phase? Which is the stationary phase?
- Which ink(s), if any, contain one dye? Which ink(s) are mixtures of dyes? Which inks contain the same dye?
Skills that are covered in the practical:
- Measuring distance travelled by solvent
- Measuring height of dye above start line (estimate to centre of spot)
- Ability to manipulate apparatus for chromatography, recording observations (e.g. number of dyes in each ink, distance travelled by solvent, height of each dye above start line)
- Ability to carry out investigation safely
Maths Skills:
- Interpreting a chromatogram
- Recording measurements accurately (1 dp)
- Substituting values correctly into expression for Rf, calculating the Rf value for the different dyes, giving answer to an appropriate number of significant figures
FOR THE DISTILLATION PART OF THE PRACTICAL:
Questions you could ask to enhance learning and focus your students on important aspects of the practical:
- Why do you need to heat the tube of ink gently?
- What is the temperature on the thermometer when the water is distilling off?
- Why does the collection tube need to be surrounded by crushed ice?
- What are the main errors in this procedure?
- How can you improve the procedure?
Skills that are covered in the practical:
- Measuring the temperature
- Ability to heat gently and safely
- Ability to manipulate apparatus for simple distillation
- Recording observations (e.g. colour of distillate)
- Ability to carry out investigation safely
Maths Skills:
- Not relevant to this part of the experiment